Organelles
Cell Membrane Structure It is present just below the cell wall in plant cells, and forms the outermost covering of an animal cell. It comprises two layers of phospholipids arranged in such a way that their hydrophobic tails are on the inner side and the hydrophilic heads form the outer side. This arrangement is called a phospholipid bilayer. Function The cell membrane provides the structure and shape to the cell, and is responsible for holding the organelles together. It regulates the entry and exit of molecules and ions from the cell, and plays a vital role in cell eating (phagocytosis) and cell drinking (pinocytosis). It is also involved in the secretion of hormones, enzymes and other molecules outside the cells, as well as in several processes of the immune system. Nucleus Structure It is the most important organelle as it contains the genetic material of the cell enclosed inside two concentric phospholipid bilayers collectively called nuclear envelope. Function The nucleus is often referred to as the 'brain' of the cell, as it controls all the activities that are carried on within the cell. It carries the information code in the form of DNA, and hence is also known as the information storage organelle. It provides a separation between DNA and the regulatory molecules of the cell, thus facilitating their interaction at the right time and stage. Ribosome Structure Ribosomes are thousands of tiny spherical structures that are made of RNA and proteins, and are present in prokaryotes as well as eukaryotes. Although ribosome is not a membrane-bound structure, it is considered to be an organelle owing to its size as well as functional importance. It is composed of two subunits which collectively form distinct binding and functional sites for tRNA molecules. Function It is the molecular machine that reads information on mRNA obtained from nucleus, and synthesizes polypeptide chains for the cell. Mitochondrion Structure This essential organelle consists of two phospholipid bilayers that form an outer membrane which encloses all the contents of the organelle; and an inner membrane which folds to form several compartments called cristae. The space between the two membranes is termed as the intermembrane (IMS) space. Function Mitochondria are the powerhouses of a cell, and are responsible for the breakdown of sugar molecules to release ATP (adenosine triphosphate), which is used to transport energy within the cell for metabolism. Hence the survival of a cell depends on the integrity of its mitochondria. Vacuole Structure Present in all plant cells and only a few animal cells, these organelles consist of a membrane called tonoplast, within which water and other molecules including organic molecules are stored. It has no particular shape or size, and reduces or enlarges according to its contents and cellular needs. Function Vacuoles function as the storehouse of a cell and store food and water, as well as waste material before it is transported outside the cell. They also provide the necessary turgor pressure against cell walls in plant cells. Chloroplast Structure A type of plant plastid, chloroplast is made up of a liquid matrix called stroma that is enclosed within two membranes termed outer and inner membranes. Dispersed into the stroma, a specialized network of membranes called thylakoids are organized into stacks called grana. These membranes contain chlorophyll and other photosynthetic pigments. Function Plants differ integrally from animals in their ability to prepare food within their cells through the process of photosynthesis. Here solar energy is harnessed by converting it into chemical energy in the form of ATP, which is then used for starch synthesis. This process of photosynthesis occurs through a set of light-dependent reactions that take place in the grana, and a set of dark (light-independent) reactions that occur in the stroma. In addition, chloroplasts are also the site of photorespiration, that involves light-dependent oxygen fixation. Cell Wall Structure A cell wall is a rigid, semi-permeable protective layer in some cell types. This outer covering is positioned next to the cell membrane. Function A major role of the cell wall is to form a framework for the cell to prevent over expansion. Cellulose fibers, structural proteins, and other polysaccharides help to maintain the shape and form of the cell. Additional functions of the cell wall include: * Support: The cell wall provides mechanical strength and support. It also controls the direction of cell growth. * Withstand turgor pressure: Turgor pressure is the force exerted against the cell wall as the contents of the cell push the plasma membrane against the cell wall. This pressure helps a plant to remain rigid and erect, but can also cause a cell to rupture. * Regulate growth: The cell wall sends signals for the cell to enter the cell cycle in order to divide and grow. * Regulate diffusion: The cell wall is porous allowing some substances, including proteins, to pass into the cell while keeping other substances out. * Communication: Cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells). * Protection: The cell wall provides a barrier to protect against plant viruses and other pathogens. It also helps to prevent water loss. * Storage: The cell wall stores carbohydrates for use in plant growth, especially in seeds. The video below provides further explanation of the organelles found in a typical plant and animal cell Well! This was a brief information regarding plant and animal cell organelles, their structure and their functions.